The Integrated Service Digital Network (ISDN)

The Integrated Service Digital Network (ISDN) was first offered to the public in the 1980s,as telephone provider began deploying their SS7 networks.With ISN, many new services could be extended to the customer premises.By using ISDN subscribers and consolidate all their trunks to one DS1 facility.The ISDN protocols provide circuit allocation within the ISDN

When additonal badwidth is needed for high speed data communication, the protocol is capable of allocating additional channel within the DS1 to carry the others calls.This is often reffered to as dynamic bandwidth allocation and is one of the principle feature of ISDN.Beside transmitting voice,ISDN is also capable of transmitting data using the same facilities as the voice

ISDN signaling uses a separates channel and is compatibel with SS7.The signaling information is handled off to the SS7 network and tranfered to the distance end using the SS7 ISDN User Part (ISUP).The ISUP was developed for all call setup adn teardown, and replaced the SS7 Telephone User Part (TUP) protocol in ANSI network

The term of ISDN was originally used to refer to the entire of IN, including SS7.This later evolved to reference only the subscriber interface.Oroginally the creator of SS7 thought of extending the SS7 network all the way to the subscribers.This was abondoned, however, over concern of security and network fraud.The solution was to create an intelligent interface, compatoble with SS7, which could offer the same services and intelligeces as the SS7 network. It was this that spuured the creation of the ISDN protocol

Perhaps the most important application for ISDN is the concept of connecting PBXs within a private network

ISDN offers many services to the subscribers.The basic lavels of service are defined as:

1.Transport elements.Allow information to be transported through the telephone service provider network and its switches, routers, multiplexer and other network equipment tranparently without alteration to the original data

2.Control elements.Support real time operations of transport capabilities (connection establishment and database queries)

3.Network managemenet elements.Provide procedures and capabilities ad administer,maintain and operate the communication infrastructure.Include provisioning of transmission facilities, fault mangement,cogestion control and administration of database and routing table

4.Communication applications environment.Provide a development for programmers from which application can be developed, using other three elements

5.Tranport.Provide the lower thredd layers of OSI, providing allocation of badwidth,routing,ralaying and error detection/correction

TO understand how ISDN can be of significant benefit to PBX networks, consider this example.Many large corporation will own several PBXs at different location.The line are often used to tie the PBXs together.This allows users to access extension in any other company location by dialling adn access number (to access teh tie line) and dialing the extension.Many digital PBXs allow callers to dial extension numbers of remote extensions without dialing an access code.Automatic routing features provide software to determine which trunk the call must be routed to

There are two classes of ISDN service: Basic Rate (BRI) and Primary Rate (PRI).BRI service provides two 64kbps bearer channel (B channels) and one 16 kbps signaling channel.This service is designed for residential and small business usage.PRI offer 23 64 kbps bearer channels (B channel) and one 64 kbps siganling channel (D channel).PRI is designed for larger businnesses with large call volume.Many PBX manufacturers already provide ISDN compatible trunking interfaces for their equimpment, making ISDN a good choice for companies who need end to end voice and data communications

ISDN cannot be succesful by itself.As we have already seen, without SS7, ISDN remains a local digital service providing a limited numbers of features and applications.With the addtion of SS7, ISDN can become an extension of the telephone network to the customer premise, offering true end to end voice and data communication woth no boundaries

The ISDN standards can be founf in the ITU-TS"I" series. The siganling standards are defined in publication Q.921 (defines the Link Access Procedure -D Channel) and Q.931 publication (defined the ISDN call control procedures at layer three)